Mayim Bialik Hospitalized in Car Accident -- 'Blossom' Actress May Lose Finger

4:35 PM PT -- MAYIM'S FINGERS ARE SAFE ... but they're not entirely functional yet -- the actress just tweeted that she "will keep all my fingers," but admitted her husband had to type the message.

Mayim Bialik sustained serious injuries in a car accident in Los Angeles this afternoon ... and sources tell us the "Big Bang Theory" actress is in danger of losing her finger.

It all went down on the corner of Hollywood Blvd. and La Brea around noon ... sources tell us the former "Blossom" actress was traveling alone in a white Volvo which was struck in the intersection by another vehicle filled with tourists from Chile.

We're told emergency responders raced to the scene. One source tells us there was "tons and tons of blood everywhere."

Sources tell us Bialik sustained major damage to her left hand ... and "her finger was almost completely severed ... it was just hanging there."

We're told 36-year-old Mayim was rushed to a nearby hospital where she is being treated.